КОЛЛОИДНЫЙ ЖУРНАЛ, 2007, том 69, № 3, с. 372-377
УДК 541.183
SYNTHESIS OF ANIONIC CORE-SHELL POLY(STYRENE-co-N-ISOPROPYLACRYLAMIDE) COLLOIDS BY A TWO STEP SURFACTANT FREE EMULSION POLYMERIZATION PROCESS
© 2007 M. Hinge
Section of Chemistry, Department of Biotechnology, Chemistry and Environmental Engineering, Aalborg University,
Sohngaardsholmsvej 57,9000 Aalborg, Denmark Поступила в редакцию 20.09.2006 г.
The aim of the performed work was to produce anionic core-shell poly(styrene-co-N-isopropylacrylamide) colloids with a N-isopropylacrylamide (NIPAM) content in the range from 5 to 30 mol %. Different batches of poly(styrene-co-NIPAM) colloids were produced with varying NIPAM mol % and the produced poly(ST-co-NIPaM) colloids were characterized by dynamic light scattering and scanning electron microscopy. Results showed that the produced colloids have a core-shell morphology with a poly(styrene) core and a poly(NIPAM) shell. The swelling ratio of the produced poly (ST-co-NIPAM) colloids was similar to the swelling ratio found for similar produced by the two step seeded polymerization process.
1. INTRODUCTION
Colloids with a solid core and a swollen thermosen-sitive shell have potential applications within many fields, such as pharmaceutical [1, 2], biotechnology [3] and environmental [4] industries. One such material is anionic poly(styrene-co-N-isopropylacrylamide) core-shell colloids (poly(ST-co-NIPAM) colloids). These colloids can be produced by a two step seeded polymerization reaction [5-8]. The principle in this synthesis is to produce and purify poly(styrene) (PS) seed particles and then in a second reaction polymerize NIPAM together with the seeds creating the poly(NIPAM) shell on the PS seeds [6-8]. This procedure is time consuming mainly due to the necessary purification of the seeds before the second synthesis step. Further, it has recently been found that the shell is not fully attached to the seed particles [9]. Another synthesis approach is to produce anionic poly(ST-co-NIPAM) colloids by a free-radical surfactant-free emulsion co-polymerization (SFECP) [5]. It has been shown by Hellweg et al. [10] that it is possible to synthesize particles by SFECP within the molar range of 0 to 100 mol % NIPAM. Comparison of the swelling ratio (a, calculated as the ratio between the swollen and collapsed diameter cubed,) of poly(ST-co-NIPAM) colloids produced by seeded [7, 8] and the SFECP [10] process, shows that the poly(ST-co-NIPAM) colloids synthesized by the two step seeded polymerization process swell twice as much at 20 mol % NIPAM and three times as much at 40 mol % NIPAM as the poly(ST-co-NIPAM) colloids synthesized by SFECP. A likely explanation to this is that in the SFECP process both NIPAM and ST monomer are incorporated into the core of the poly(ST-co-NIPAM) colloids. NIPAM incorporated into the hydro-phobic PS rich core will be excluded from the surrounding water and will therefore not be able to swell.
This explanation is supported by Hellweg et al. [10] who shows that there was no significant swelling of the poly(ST-co-NIPAM) colloids with a NIPAM content < 25 mol %. It was also stated that the hardness of the poly(ST-co-NIPAM) colloids decreased with increasing NIPAM mol % content in the range up to 25 mol % NIPAM [10] which again supports NIPAM is incorporated into the PS core. This means that at present it is only possible to produce anionic core-shell poly(ST-co-NIPAM) colloids with NIPAM content below 25 mol % by the two step seeded polymerization process. This polymerization process is time consuming and the poly(NIPAM) shell is not fully attached to the PS core material. Thus, no satisfactory synthesis path exists to produce anionic poly(ST-co-NIPAM) colloids with a NIPAM content in the range from 5 to 30 mol % that have a hairy core-shell morphology.
A potential solution might be to synthesize ST by a two step free-radical surfactant-free emulsion polymerization (SFEP) where the second step is an addition of NIPAM to the growing ST colloids. It has been shown by Duracher et al. [11] that it is possible to synthesize cationic poly(ST/NIPAM/aminoethylmethacrylate) core-shell latex particles by a two step SFECP. The first step in the two step SFECP was initiating a SFECP of ST and NIPAM with 2,2*-azobis (2-amidinopropane) dihydrogenchloride (V50). In the second step NIPAM, aminoethylmethacry-late, methylenebisacrylamide and more initiator was added to the synthesis. The addition were done at different times after initiation (different conversions of the "seeds") in order to determine the optimum time (after 150 min and 50% conversion) for the addition. Cationic poly(ST-co-NIPAM) colloids were also synthesized without a second addition of monomer. Scanning electron microscopy (SEM) of colloids produced by the SFECP of ST and NIPAM alone showed a "raspberry-like" surface mor-
phology. It was also shown that the NIPAM monomer was consumed before the ST monomer during this synthesis and it was argued that the poly(NIPAM) patches on the surface were due to demixing of the two polymers during synthesis. The work of Duracher et al. [ll] thereby shows that it is possible to produce poly(ST-co-NIPAM) colloids by a SFECP process, although the resulting colloids do not have a smooth surface and they are cationic.
In this work it will be attempted to synthesize anionic poly(ST-co-NIPAM) colloids with a NIPAM content in the range from 5 to 30 mol % by a two step SFEP. These colloids will be produced by addition of NIPAM to a SFEP of ST using potassium persulphate (PPS) as initiator. The reason for delaying the addition of the comonomer is to avoid incorporation of NIPAM into the core as seen for the anionic poly(ST-co-NIPAM) colloids and the "raspberry-like" surface morphology seen for the cationic poly(ST-co-NIPAM) colloids. It is due to the differences in initiator and monomer composition not possible to adopt the optimal delay-time found in [11]. It has been shown that during a SFECP of ST and acrylic acid (AA) using PPS as initiator a shift in polymerization locus from the PS rich core to the AA rich shell takes place around 60 min after initiation [12]. The attempted synthesis use PPS and ST in the same molar ratios as applied in the above SFECP of ST and AA. It is therefore assumed that a shift also will happen around the same time in the attempted syntheses. This means that it is important that the NIPAM monomer is present in the synthesis after the "seeds" are generated and before the shift in polymerization locus occurs. The addition of NIPAM to the SFEP of ST will therefore in this work be done 30 min after initiation. This paper presents a new way to synthesize an-ionic core-shell poly(ST-co-NIPAM) colloids with a NIPAM content in the range from 5 to 30 mol %. The produced poly(ST-co-NIPAM) colloids are characterized by dynamic light scattering and SEM.
2. EXPERIMENTAL 2.1. Materials
Poly(ST-co-NIPAM) colloids were synthesized by a two step SFEP using PPS (Merck, pro. analysi) as initiator, styrene (ST) (Acros Organic, 99%) as monomer and N-isopropylacrylamide (NIAPM) (Aldrich, 97%) as co-monomer. The water used in the synthesis was ultrapure water from a Milipore water polishing system (18 MQcm). The inhibitor tert-butylcatechol was removed from ST by an Al2O3 based inhibitor remover (Sigma-Aldrich).
2.2. Preparation procedure
The syntheses were performed in a 2 l round bottomed flask with a reflux condenser, nitrogen inlet, and a stir paddle. Throughout the synthesis the temperature was held constant at 70°C using a water bath. The 1150 ml water used in the synthesis was preheated and degassed by
flushing the synthesis flask with nitrogen while stirring at 350 RPM for 1 hour, prior to addition of regents. The inhibitor was removed from ST by stirring for 1 hour with 10 g/l inhibitor remover at ambient temperature. ST was then filtered first through a Whatmann 113 and then through a Whatmann 41 filter. NIPAM and PPS were used as received. ST was added and heated for around 5 min before addition of initiator (dissolved in 50 ml water) and after 30 min NIPAM was added to the synthesis. Both stirring and nitrogen flow continued throughout the synthesis. Table 1 gives the identification and formulation details for the different batches of poly(ST-co-NIPAM) colloids.
The synthesized poly(ST-co-NIPAM) colloids were purified by dialysis against deionized water (sample to water ratio, 1 : 15) for two weeks, until the sample conductivity was below 100 ^S/cm, (water changed every second day) at ambient temperature using a dialysis tube with a molecular cut off of 12-16 kD (Visking).
2.3. Characterization
The monomers have a relative high vapor pressure and they will therefore evaporate together with the solvent during drying. It is therefore possible to determine the yield gravimetrically after synthesis and dialysis. Two samples of 10 ml (Vs) were dried in aluminium pans and the mass of polymer were determined gravi-metrically after drying for 24 hr at 104°C. The yield is then calculated from the average polymer mass (mp) by following equation
Yield =
m
add s
(1)
where madd is the mass of monomer and initiator added in the synthesis feed and Vtot is the total post synthesis and post dialysis volume.
The hydrodynamic diameter of the produced poly(ST-co-NIPAM) colloids was determined by dynamic light scattering (DLS) (Zetamaster, Malvern Instruments) using a 100 mm quarts cuvette. Poly(NIPAM) have a critical temperature of 32°C [13]. Determining the hydrodynamic diameter of the poly(ST-co-NIPAM) colloids below and above the critical temperature gives the swollen and collapsed diameter of the poly(ST-co-NIPAM) colloids respectively. The hydrody-namic diameter of the poly(ST-co-NIPAM) colloids was determined at 18 and 60°C to obtain the swollen and collapsed diameter of the poly(ST-co-NIPAM) colloids.
SEM images of the produced poly(ST-co-NIPAM) colloids (except for ST95/05
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